Among all cardiovascular-related diseases, coronary artery disease still remains the leading cause of death in western countries. The incidence and progression of heart disease is markedly different in males and females, indicating that sex-biased factors can protect from disease. The long term objectives of this project are to identify processes, regulated by sex chromosomes and gonadal hormones, that affect cardiovascular disease, to improve the understanding of endogenous mechanisms of disease, and to identify sex-biased protective factors that may become targets for therapies. The project utilizes novel mouse models, which have already provided new evidence for striking differences in the response of XX and XY mice to myocardial ischemia/reperfusion injury, independent of the gonadal sex of the mice. These novel models vary the number or type of sex chromosomes in mice that have the same type of gonad, and thus allow the first understanding of the differential effects of XX vs. XY chromosomes. XX mice show dramatically greater susceptibility to ischemia/reperfusion injury, relative to XY mice, and lower post-ischemic heart contractile function. The XX vs. XY difference is attributable to the number of X chromosomes, not the presence/absence of the Y chromosome. Thus, the X chromosome harbors factors that strongly influence ischemia/reperfusion injury in a dose-dependent and sexually biased manner. Aim 1 is to investigate the physiological and molecular mechanisms that account for the XX vs. XY difference using these mouse models by measuring heart functional recovery and infarct size, mitochondrial function, superoxide production, and protective signal transduction pathways. Aim 2 is to first identify a list of candidate X gene(s) responsible for the XX vs. XY difference, and then to test the role of specific candidate genes by manipulating their expression in vivo. . Response to ischemia/reperfusion injury will be measured in mice with different doses of specific candidate X genes. Aim 3 is to manipulate the levels of adult gonadal hormones and type of gonad to understand how estrogens and androgens act on XX and XY mice to cause protection from ischemia/reperfusion injury. Response to ischemic insult will be measured as a function of hormonal level, sex chromosome complement and age. Discriminating the hormonal vs. chromosomal consequences of differences between females and males will provide an essential foundation for understanding factors that protect from ischemia/reperfusion injury, with an eye toward harnessing the protective factors to develop novel therapies.